Engineered E2 For Increasing The Content Of Free LYS11-Linked Ubiquitin

ABSTRACT

The invention provides a chimeric E2 enzyme comprising a Ubc domain fused to a heterologous ubiquitin binding domain (UBD). The chimeric enzymes of the invention may be useful in producing elevated levels of free polyubiquitin.

RELATED APPLICATIONS AND INCORPORATION BY REFERENCE

This application is divisional of U.S. Ser. No. 13/670,594 filed Nov. 7, 2012, which is a continuation-in-part application of international patent application Serial No. PCT/GB2011/000704 filed 6 May 2011, which published as PCT Publication No. WO 2011/138593 on 10 Nov. 2011, which claims benefit of GB patent application Serial No. 1007704.8 filed 7 May 2010 and U.S. provisional patent application Ser. No. 61/333,145 filed 10 May 2010.

The foregoing applications, and all documents cited therein or during their prosecution (“appln cited documents”) and all documents cited or referenced in the appln cited documents, and all documents cited or referenced herein (“herein cited documents”), and all documents cited or referenced in herein cited documents, together with any manufacturer's instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention. More specifically, all referenced documents are incorporated by reference to the same extent as if each individual document was specifically and individually indicated to be incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to engineered E2 ubiquitin conjugating enzymes. In particular, the invention relates to chimeric E2 enzymes which are fused to a ubiquitin binding domain (UBD). The fusion is engineered by replacing the c-terminal tail of a class II E2 enzyme with a UBD, such that the Ubiquitin conjugating (Ubc) catalytic domain is fused to the UBD. This modification increases the efficiency of ubiquitin polymerisation by E2 enzymes, and facilitates isolation of specific forms of polyubiquitin.

BACKGROUND OF THE INVENTION

Protein ubiquitination is a versatile posttranslational modification with roles in protein degradation, cell signaling, intracellular trafficking and the DNA damage response (Chen and Sun, 2009; Komander, 2009). Ubiquitin polymers are linked through one of seven internal lysine (K) residues or through the N-terminal amino group. Importantly, the type of ubiquitin linkage determines the functional outcome of the modification (Komander, 2009). The best-studied ubiquitin polymers, K48- and K63-linked chains, have degradative and non-degradative roles, respectively (Chen and Sun, 2009; Hershko and Ciechanover, 1998). However, recent data has revealed an unexpected high abundance of so-called atypical ubiquitin chains; for example, K11 linkages have been found to be as abundant as K48-linkages in S. cerevisiae (Peng et al., 2003; Xu et al., 2009).

Polyubiquitin chains are assembled on substrates through the concerted action of a three-step enzymatic cascade, involving an E1 ubiquitin activating enzyme, an E2 ubiquitin conjugating enzyme, and E3 ubiquitin ligases (Dye and Schulman, 2007). While E3 ligases attach polyubiquitin chains to a target and thus confer substrate specificity, E2 enzymes are thought to determine the type of chain linkage in polyubiquitin chains. K48- and K63-specific E2 enzymes have been identified (Chen and Pickart, 1990; Hofmann and Pickart, 1999), which allowed structural analysis of these chain types as well as a detailed understanding of specificity of ubiquitin binding domains (UBDs) and deubiquitinases (DUBs) (reviewed in Komander, 2009). This information is currently lacking for atypical ubiquitin chains.

Several recent reports have implicated K11-linked ubiquitin chains in distinct biological processes. Early data indicated that K11-linked chains are proteasomal degradation signals (Baboshina and Haas, 1996). An E2 enzyme, UBE2S/E2-EPF, was identified that assembled K11 linkages in vitro (Baboshina and Haas, 1996). The human anaphase promoting complex (APC/C) was found to assemble K11 linkages using the E2 enzyme UBE2C/UbcH10, on proteins that need to be degraded for cell cycle progression (Jin et al., 2008). A yeast proteomics study, apart from having revealed the high abundance of K11 linkages, also implicated this chain type with endoplasmic reticulum-associated degradation (ERAD), and identified yeast Ubc6 as an E2 enzyme involved in synthesis of K11-linked chains (Xu et al., 2009). In mammalian cells, K11 linkages were found to be enriched in UBA/UBX protein complexes, which interact with the key ERAD regulator p97/cdc48 (Alexandru et al., 2008). Hence, K11-linked chains seem to regulate numerous important cellular processes, and may act as a distinct proteasomal degradation signal. However, cellular mechanisms of assembly and disassembly of K11 linkages, as well as structural determinants for K11 linkage recognition, are unknown.

The structure of E2 enzymes is well characterised. All E2 enzymes comprise a conserved domain of about 16 kD (the Ubc domain) which contains the Ubc motif, [FYWLS]-H-[PC]-[NH]-[LIV]-x(3,4)-G-x-[LIV]-C-[LIV]-x-[LIV]. The Ubc domain contains a conserved cysteine residue, which accepts ubiquitin from the ubiquitin-activating enzyme E1 to form a thiol ester. Substitution of the conserved cysteine abolishes E2 activity. A suggested motif rich in basic residues is found at the N-terminus of the UBC domain which may be involved in E1 binding.

E2 enzymes can be classified on the basis of their structure into three classes.

Class I: these proteins comprise simply the “Ubc” catalytic domain. In vitro these enzymes are very poor at transferring ubiquitin to proteins on their own, and probably require an E3 to aid this in vivo. UBC 4 and 5 of S. cerevisiae, UBC1 of Arabidopsis thaliana, and human UBE2D1, UBE2D2, UBE2D3 or UBE2D4 are examples of this class of E2, and are known to be important in the ubiquitination of many short-lived and abnormal proteins prior to degradation.

Class II: these enzymes contain a C-terminal tail attached to the Ubc domain. The tails are different in type but very acidic tails, as found in Ubc2 (also known as Rad6) of S. cerevisiae, appear to mediate interaction with protein substrates, in this case with the basic histones. Ubc2/Rad6 will ubiquitinate histones in vitro, which requires the C-terminal tail and is known to be involved in DNA repair. This may be a form of ubiquitination that results in protein modification but not degradation. Other C-terminal tails appear to be involved in E2 localisation. Ubc6 of S. cerevisiae is found anchored to the ER membrane with the active site facing the cytosol. The 95 residue C-terminal tail of Ubc6 includes a hydrophobic signal-anchor sequence.

Class III: N-terminal extensions are present in this class of E2s. Several enzymes of this class have been identified but the function of the extensions is unknown.

Ubiquitin binding domains are modular protein elements that bind non-covalently to ubiquitin. They are typically small, being 20 to 150 amino acids in length, and independently-folded, making their isolation straight forward. They are based on a number of different ubiquitin binding motifs. The Ubc of E2 enzymes is one class of ubiquitin binding domain (UBD). Other classes include α-helical domains, zinc finger domains (ZnFs) and plekstrin homology (PH) domains. See, for example, Dikic et al., 2009. Many UBDs are known in the art; for example, see Table 1 in Dikic et al., page 663.

Isopeptidase T (IsoT, or USP5) contains a ZnF-type UBD (known as ZnF UBP or PAZ domain) between amino acid positions 163 and 291 (see Reyes-Turcu et al., 2006). HDAC6 (Boyault et al., 2006) also contains a ZnF UBP domain. Other zinc finger ubiquitin binding domains include UBZ domains, as contained in polymerase-h and polymerase-k; NZF and A20-like ZnF domains.

Alpha-helical types of domains include, for example, UBA domains, found in Rad23 and R23A proteins, or ubiquitin interacting motifs (UIM, MIU or dUIM); see Dikic et al., 2009.

The study of the ubiquitin system requires the ability to produce unattached polymeric ubiquitin in solution, for structural and functional analysis. As noted above, ubiquitin chains vary according to which of the 7 internal Lys residues is used for concatenation of the ubiquitin molecules. In absence of a E3 ubiquitin ligase, most E2 enzymes fail to assemble polyubiquitin. Class II E2 enzymes can assemble polyubiquitin chains on their own C-terminal tails. Very few E2 enzymes, including UBE2R2/cdc34, UBE2K and UBE2S produce free, i.e. unattached, polyubiquitin in solution. For instance, UBE2S, which assembles K-11 linked polyubiquitin, is inefficient at producing free ubiquitin multimers in solution, producing only small amounts of free ubiquitin dimers. There is a need, therefore, for improved E2 enzymes that can be used to produce free polyubiquitin in solution.

Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.

SUMMARY OF THE INVENTION

Applicants have analyzed the K11-specific E2 enzyme UBE2S that assembles K11-linked chains on its own C-terminal tail in vitro, and also generates limited amounts of free, i.e. unattached, K11-linked diubiquitin. By removing the C-terminal tail, Applicants have engineered an E2 enzyme that produces free K−1 linked diubiquitin. Furthermore, by replacing the C-terminal tail of the E2 with a UBD, Applicants have engineered a UBE2S fusion protein that synthesizes free K11-linked polymers, including trimers and tetramers, with markedly increased efficiency, allowing high-level purification of K11-linked ubiquitin dimers, trimers and tetramers, and facilitating structural studies.

In a first aspect, therefore, there is provided an E2 enzyme comprising a Ubc domain, from which an N-terminal or a C-terminal tail has been removed.

In a preferred embodiment, the Ubc domain is fused to a heterologous ubiquitin binding domain (UBD).

Preferably, the UBD is C-terminal to the Ubc domain. In class II E2 enzymes, a C-terminal amino acid extension is present, which is partly replaced by the UBD. Some E2 enzymes, such as class Ill enzymes, have an N-terminal tail which may be removed and optionally at least partly replaced with a UBD.

UBDs are known in the art, and exemplary UBDs may be of the α-helical, zinc finger or plekstrin homology domain classes.

For example, the UBD is a domain selected from the group consisting of UIM, IUIM (MIU), DUIM, UBM, UBA, GAT, CUE, VHS, UBZ, NZF, ZnF A20, ZnF UBP (PAZ), PRU, GLUE, UEV, UBC, SH3, PFU and Jab1/MNP domains.

Preferably, the UBD is a ZnF UBP domain, such as the UBD derived from Isopeptidase T. It advantageously may comprise the sequence from about position 163 to about position 291 of Isopeptidase T, which may comprise the UBD. For example, it may comprise residues 173-289 of Isopeptidase T.

Alternative UBDs include UBA, UIM and NZF domains.

ZnF and NZF domains are particularly preferred.

The Ubc will determine the specificity of linkages used in the polyubiquitin chains. Ubc domains may be derived from E2 enzymes. Referring to human E2 enzymes, the Ubc domain may be derived from an E2 enzyme selected from the group consisting of UBE2A, UBE2B, UBE2C, UBE2D1, UBE2D2, UBE2D3, UBE2D4, UBE2E1, UBE2E2, UBE2E3, UBE2F, UBE2G1, UBE2G2, UBE2H, UBE2I, UBE2J1, UBE2J2, UBE2K, UBE2L3, UBE2L6, UBE2M, UBE2N, UBE2NL, UBE2O, UBE2Q1, UBE2Q2, UBE2R1, UBE2R2, UBE2S, UBE2T, UBE2U, UBE2W, UBE2Z, and BIRC6. The foregoing are human E2 enzymes. Of course, mammalian, yeast or other E2 enzymes may be used, preferably those enzymes which are equivalent to the foregoing human enzymes.

Preferably, the Ube domain is derived from UBE2S. The Ubc domain is comprised in residues 1 to 156 of UBE2S, and advantageously these residues are incorporated into the chimeric E2 enzyme. Residues 196-222 of UBE2S comprise the C-terminal extension; these residues are removed and/or replaced with a UBD.

The invention provides a method for increasing the capacity of an E2 enzyme to produce free polyubiquitin dimers, comprising removing a C-terminal tail from said E2 enzyme. In a further aspect, the invention provides a method for increasing the capacity of an E2 enzyme to produce free polyubiquitin chains containing more than two ubiquitin monomers in solution, comprising conjugating the Ubc domain of said E2 enzyme to a UBD.

Preferably, the polyubiquitin chains comprise trimers or tetramers of ubiquitin monomers.

In a further aspect, the invention provides a method for producing free polyubiquitin chains linked through a desired lysine residue, comprising the steps of: (a) selecting an E2 enzyme which possesses the desired specificity for ubiquitin lysine residues; (b) fusing the Ubc catalytic domain of said E2 enzyme to a UBD ubiquitin binding domain; and incubating the resulting chimeric protein with an E1 ubiquitin activating enzyme and monomeric ubiquitin.

In a preferred embodiment, the incidence of undesired lysine linkages is reduced by including a linkage-specific deubiquitinase in the incubation mixture. Such enzymes preferentially degrade polymers having a specific lysine linkage; thus, if the product of the chimeric E2 is contaminated with undesired linkage polymers, the contaminants may be specifically removed.

Accordingly, it is an object of the invention to not encompass within the invention any previously known product, process of making the product, or method of using the product such that Applicants reserve the right and hereby disclose a disclaimer of any previously known product, process, or method. It is further noted that the invention does not intend to encompass within the scope of the invention any product, process, or making of the product or method of using the product, which does not meet the written description and enablement requirements of the USPTO (35 U.S.C. §112, first paragraph) or the EPO (Article 83 of the EPC), such that Applicants reserve the right and hereby disclose a disclaimer of any previously described product, process of making the product, or method of using the product.

It is noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises”, “comprised”, “comprising” and the like can have the meaning attributed to it in U.S. patent law; e.g., they can mean “includes”, “included”, “including”, and the like; and that terms such as “consisting essentially of” and “consists essentially of” have the meaning ascribed to them in U.S. patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the invention.

These and other embodiments are disclosed or are obvious from and encompassed by, the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example, but not intended to limit the invention solely to the specific embodiments described, may best be understood in conjunction with the accompanying drawings.

FIG. 1: UBE2S is a K11-specific E2 enzyme. (a) UBE2S and (b) UBE2C were analyzed in autoubiquitination assays in the presence of E1, ubiquitin and Mg.ATP. The panel of single-Lys ubiquitin mutants reveals the intrinsic linkage specificity. Autoubiquitination is visualized with a polyclonal anti-ubiquitin antibody. UBE2S, but not UBE2C, autoubiquitinates and also assembles unattached K11-linked ubiquitin chains. (c) Time course assay for autoubiquitination by UBE2S. The reaction for wild-type (wt) and K11-only ubiquitin leads to similar high-molecular weight conjugates, while for the Lysless (K0) and K63-only ubiquitin an equivalent pattern of multimonoubiquitination is observed.

FIG. 2: Assembly of K11-linked diubiquitin. (a) Domain structure of UBE2S, and autoubiquitination reactions with UBE2S wild-type and catalytic mutants. (b) UBE2S autoubiquitination occurs in cis. Wild-type UBE2S was mixed with GST-tagged inactive UBE2S^(C95A), and after precipitation of the GST-tagged protein, ubiquitination in supernatant (left) and precipitate (right) is analyzed. (c) Removal of the Lys-rich tail of UBE2S decreases autoubiquitination while preserving K11 specificity. (d) Purification of K11-linked diubiquitin by cation exchange chromatography. The integrated peak area (mAU*ml) is indicated. A gel showing protein-containing fractions is shown as an inset.

FIG. 3: Assembly of K11-linked tetraubiquitin. (a) UBE2S engineering to increase yields of free K11-linked ubiquitin chains. The C-terminal tail was replaced with the ZnF-UBP domain of USP5/IsoT. The fusion protein assembles free chains of up to five ubiquitin molecules, yet it is less specific and also incorporates K63-linkages with wild-type and K63-only ubiquitin (indicated by arrows). (b) Incorporation of K63-linkages may be counteracted by using a K63R ubiquitin mutant, or by including the K63-specific DUB AMSH in the reaction, as observed by disappearance of the faster migrating K63-linkage contamination. (c) 5 μl aliquot of a 1 ml chain assembly reaction using 25 mg ubiquitin shows that di-, tri- and tetraubiquitin is generated in milligram quantities. (d) Cation exchange chromatography was used to purify K11-linked ubiquitin chains. The integrated peak area (mAU*ml) is specified. A gel showing protein-containing fractions is shown as an inset. (e) Purified ubiquitin tetramers of K11, K48, K63 and linear linkages have different electrophoretic mobility on 4-12% SDS-PAGE gels.

FIG. 4: Crystal structure of K11-linked diubiquitin. (a) The crystal structure of K11-linked diubiquitin in two orientations. The proximal (orange) and distal (yellow) molecules interact through the ubiquitin helix, and the isopeptide linkage (shown in ball-and-stick representation, with red oxygen and blue nitrogen atoms) is at the surface of the dimer. (b) A semitransparent surface coloured blue for residues Ile44, Leu8 and Val70 shows that the hydrophobic patch is not involved in the interface. (c) Residues at the interface are shown in stick representation, and polar interactions of <3.5 Å are shown with dotted lines. Water molecules are shown as purple spheres. (d) The hydrophobic surface in K11-linked chains is extended by Leu71 and Leu73, which are exposed as Arg72/Arg74 participate in the interface.

FIG. 5: NMR Solution studies of K11-linked diubiquitin.

(a) Overlay of 15N, 1H HSQC spectra of ubiquitin K63R (red) onto K11-linked diubiquitin K63R (blue). The expansion illustrates the doubling of peaks observed for Lys29, Ile30, Asp32 and Lys33. The signal for Asp52 is unperturbed. (b, c) Weighted chemical shift perturbation according to residue number for K11-linked diubiquitin with both molecules ¹³C, ¹⁵N-labeled (blue, K63R ubiquitin mutant) or only labeled distally (orange, K11R ubiquitin mutant). Shown are chemical shift perturbations observed for doubled peaks calculated as the weighted difference between the chemical shift position in the K11-linked diubiquitin mutants and their respective monoubiquitin counterparts at pH7.4 (b) and pH 3.5 (c). Stars (*) indicate exchange-broadened residues, and arrows indicate K29 and K33. (d) Combined chemical shift perturbation differences for K48- and K63-linked diubiquitin (Tenno et al., 2004). (e) Comparison of the proximal K11-linked diubiquitin interface in a view indicated by the arrow (left). Surface map of interacting residues from NMR (middle, orange, with shifting residues in blue, and Pro residues in yellow) and from the crystal interaction (right, yellow with interface resides in marine, according to the PISA server, ebi.ac.uk/pdbe/prot_int/pistart.html). (f) Comparison of the distal K11 diubiquitin interface, coloured as in (c), as viewed indicated by the arrow in the left picture. The 2nd image shows perturbed residues obtained from the distally labeled sample, and the third image from the fully labeled diubiquitin. The fourth image corresponds to the crystal structure interface. Asp39 and Glu52, which form part of the crystallographic interface but are not perturbed in solution, are circled. A white surface indicates exchange-broadened residues.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art, such as in the arts of peptide chemistry, cell culture and phage display, nucleic acid chemistry and biochemistry. Standard techniques are used for molecular biology, genetic and biochemical methods (see Sambrook et al., Molecular Cloning: A Laboratory Manual, 3rd ed., 2001, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.; Ausubel et al., Short Protocols in Molecular Biology (1999) 4th ed., John Wiley & Sons, Inc.). All publications cited herein are incorporated herein by reference in their entirety for the purpose of describing and disclosing the methodologies, reagents, and tools reported in the publications that might be used in connection with the invention.

E2 enzymes, as referred to herein, are variously known as ubiquitin carrier proteins, ubiquitin conjugating enzymes or Ubcs. In many instances, E2 enzymes are thought to determine linkage specificity in polyubiquitin. 38 E2 enzymes have been identified in humans, as described in Ye and Rape, 2009. As noted above, they may be subdivided into three classes, of which class II enzymes have a C-terminal extension or tail attached to the Ube catalytic domain (also referred to as the UBCc catalytic domain). This domain is recognised as a conserved domain, and is identifiable in any E2 enzyme.

Ubiquitin binding domains, or UBDs, are modular protein domains which bind non-covalently to ubiquitin. As noted above, UBDs are divisible into a number of different categories, including α-helical, zinc finger and pleckstrin homology domains, which are structurally diverse. Preferably, a UBD is a UBD as described in Dikic et al., 2009. Other UBDs may become recognised, and it is anticipated that these too will be useful in the present invention. In one embodiment, a UBD is a ZnF UBD, for example UBZ, NZF, A20-like ZnF or ZnF UBP, as described in Dikic et al., 2009.

A chimeric protein may be constructed by fusing a Ubc domain to a UBD, according to techniques known in the art. For example, polypeptide fusions may be created by ligating nucleic acids encoding the respective domains in-frame, and expressing the coding sequence thus created. The domains may be fused directly to one another, or may be separated by one or several additional amino acids, referred to as a linker. Where a linker separates the domains, said linker advantageously does not negatively influence the three-dimensional alignment of the domains in such a way that their functional cooperation is sterically hindered. The UBD is preferably C-terminal to the Ubc domain, effectively replacing the C-terminal extension in a Class II E2.

A chimeric enzyme is an enzyme that may comprise at least two heterologous domains. In this context, heterologous signifies that the domains are not found in the same position in a single polypeptide in vivo. Normally, this means that the domains are derived from two different proteins. The proteins themselves may be found in the same organism—for example, the proteins may both be human proteins.

The term “fusion protein” refers to a protein or polypeptide that has an amino acid sequence derived from two or more proteins, for example two heterologous domains as indicated above. The fusion protein may also include linking regions of amino acids between amino acid portions derived from separate proteins. Unrelated proteins or polypeptides may also be included in the fusion, for example immunoglobulin peptides, dimerising polypeptides, stabilizing polypeptides, amphiphilic peptides, or polypeptides which may comprise amino acid sequences that provide “tags” for targeting or purification of the protein.

In one embodiment, a chimeric enzyme may also be an enzyme in which the positioning, spacing or function of two endogenous domains has been changed, by manipulation, with respect to the wild-type enzyme. For example, a C-terminal extension in a class II E2 may be repositioned by adding or removing amino acids between it and the Ubc domain. Alternatively, the amino acid sequence of the C-terminal extension itself may be mutated, to introduce desired properties. Typically, such properties include the ability to bind ubiquitin.

A protein domain, as referred to herein, is a protein or fragment of a protein which is capable of independent folding to create a defined three-dimensional structure that imparts a property to the domain. Typically, the domain is identified by its amino acid sequence, usually by identifying certain limits in a protein structure which define the domain. Domains may be identified using domain databases such as, but not limited to PFAM, PRODOM, PROSITE, BLOCKS, PRINTS, SBASE, ISREC PROFILES, SAMRT, and PROCLASS. It will be understood that the precise limits of the domain, as defined by the amino acid sequence, may vary. For example, including extra amino acids which are not normally considered to be part of the domain is unlikely to affect the function of the domain. The use of interdomain linkers is commonplace in the art to link protein domains, both in nature and in artificial protein constructs. Such linkers typically comprise sequences present upstream or downstream of the joined domains in their natural context. Moreover, removing one or more amino acids from one end of a domain may be permissible, as long as a substantial part of the domain remains which is still able to fold in the correct manner to mediate the desired function. In one embodiment, therefore, a domain is a minimal independently-folding segment of a protein which possesses the desired functional characteristic. In the case of the Ubc domain, this function is the polymerisation of ubiquitin using the desired lysine linkage. In the case of the UBD, the function is to promote the formation of free ubiquitin polymers.

In one embodiment, the entire sequence of a domain as defined by primary amino acid sequence is used. In another embodiment, a sequence shortened by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20 or more amino acids at the N and/or C terminus may be used.

The present invention increases the amount of free polyubiquitin produced by E2 enzymes, making the polyubiquitin available for any desired purpose. In this context, the production of free polyubiquitin may be increased by 10%, 15%, 20%, 25%, 50%, 75%, 100% or more. Free polyubiquitin refers to polyubiquitin chains, for example dimers, trimers, tetramers or longer chains, released into solution by the E2 enzyme rather than attached to a target.

A “nucleic acid” is a polynucleotide such as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The term is used to include single-stranded nucleic acids, double-stranded nucleic acids, and RNA and DNA made from nucleotide or nucleoside analogues.

Ubc (UBCc) domains of E2 proteins share a consensus sequence, 141 amino acids in length. Comparison of Ubc domains suggests a consensus sequence

[SEQ ID no 19] SKRLQKELKDLKKDPPSGIS AEPVEENLLEWHGTIR  GPPDTPYEGGIFKLDIEFP EDYPFKPPKVRFVTKI YHPPNVDENG  KICLSI LKTHGWSPAY TLRTVLLSLQSLLN EPNPSDPLNAEAAK  LYKENREEFKKKAREWT. The Ubc motif, [FYWLS]-H-[PC]-[NH]-[LIV]-x(3,4)-G-x-[LIV]-C-[LIV]-x-[LIV], is underlined [SEQ ID no 20]. Preferably, the Ubc domain used in the present invention conforms to the consensus sequence, allowing for conservative amino acid substitutions. Substitutions to the conserved sequence may also be made which reflect deviation from the consensus seen in naturally-occurring Ubc domains. Therefore, the Ubc domain used in the present invention may be naturally occurring or synthetic. Synthetic domains may be designed according to the above consensus and constraints.

Naturally-occurring Ubc domains may be derived from proteins other than E2 enzymes.

Conservative amino acid substitutions generally follow the following scheme:

Side chain Members Hydrophobic met, ala, val, leu, ile Neutral hydrophilic cys, ser, thr Acidic asp, glu Basic asn, gln, his, lys, arg Residues that influence chain gly, pro orientation Aromatic trp, tyr, phe

In the above table, amino acids identified in the same row are considered to have similar side-chains and may be substituted for each other with the least impact on protein structure and function.

A list of known E2 enzymes, identified by human gene names together with yeast homologue names, appears in Table S1 in the supplementary information supplied with Ye & Rape, 2009. In the context of the present invention, E2 enzymes may be selected from this list, and Ubc domains derived therefrom for use in constructing chimeric E2 enzymes.

For example, in order to improve the production of free Lys-11 conjugated polyubiquitin, UBE2C or UBE2S should be employed. In UBE2C the Ube domain extends from residue 33 to residue 170 of the amino acid sequence.

In order to produce Lys-48 chains, UBE2G1, UBE2G2, UBE2K, UBE2R1 or UBE2R2 may be used. In UBE2G1, for example, the Ubc domain is located between residues 74 and 216 of the amino acid sequence.

Other E2 enzymes, and reported chain specificities, are set forth in Ye and Rape, 2009, as mentioned above.

Table 1 shows Seq IDs 1 to 13, which set forth exemplary nucleotide and amino acid sequences of human E2 enzymes, and identify the Ubc (UBCc) domains therein. Other sequences are available in databases, such as SWISSPROT, TrEMBL, NCBI, and the like.

TABLE 1 Name SEQ ID UBCc position UBE2C 1 33-170 UBE2D1 2  4-142 UBE2D2 3  4-142 UBE2D3 4  4-142 UBE2E2 5 59-196 UBE2E3 6 65-202 UBE2F 7 35-180 UBE2J1 8 12-119 UBE2J2 9 14-127 UBE2M 10 33-166 UBE2N 11  5-144 UBE2O 12 958-1108 UBE2S 13 13-152

Ube domains may be obtained from the sequences set forth above, or other E2 sequences known in the art, and covalently linked to UBD domains to create a chimeric protein. Alternatively, nucleic acids encoding domains suitable for generating chimeric E2 enzymes may be produced, for example, by restriction enzyme digestion of nucleic acids encoding the desired E2 enzyme, or by PCR amplification of a desired nucleic acid sequence using primers that flank the Ubc domain. Nucleic acids encoding E2 enzymes are known in the art and sequences therefore widely available in databases such as GENBANK. Restriction enzyme cutting sites and suitable primers may be identified using suitable software, or by eye.

The invention contemplates the use of natural Ubc domains that have been mutated. Mutation may be at the nucleic acid level, that is changes may be effected to the nucleic acid encoding a Ubc domain without changing the structure of the Ubc domain itself, as a result of redundancy in the genetic code. Such changes may, for example, confer improved expression in heterologous host cells by employing preferred codon usage patterns.

Other mutations will change the amino acid sequence of the Ubc domain. As noted above, this may take the form of additions to or deletions from the N and C termini of the domain. Moreover, changes may be made within the sequence of the Ubc domain, for example through substitution, addition or deletion of one or more amino acids. Conservative amino acid substitutions are preferred, as set forth above. For example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20 or more amino acids are added, deleted and/or substituted by other amino acids.

In a preferred embodiment, the naturally occurring Ubc sequence is used.

Expression of nucleic acids encoding chimeric E2 enzymes may be carried out in any suitable expression system. Expression systems are known in the art and may be obtained commercially or according to instructions provided in laboratory manuals.

More than 20 families of UBD have been identified. The first UBD to be identified was from S5a, a proteasome subunit, and this sequence was used in bioinformatic analyses to identify further domains, which were then shown to be bona-fide UBDs. A pattern was identified, known as the ubiquiting-interacting motif (UIM). A second motif, the Ubiquitin-Associated domain (UBA), was first identified as a domain common to proteins involved in ubiquitin metabolism. This domain too was shown to bind ubiquitin. Further domains have been discovered, including CUE domains, which are associated with Endoplasmic Reticulum targeting, and the zinc finger NZF or PAZ domains, VHS and GLUE domains.

The UEV domain is a pleckstrin homology UBD which resembles s Ubc domain, but lacks the catalytic cysteine residue. For a description of UBD domains, see Hicke et al., 2005, and Dikic et al., 2009, especially Table 1 in the latter document.

UBDs useful in the present invention may be obtained from naturally-occurring polypeptides, or may be mutated forms of domains present in such polypeptides. As noted above, mutant proteins may be created by inserting, deleting or substituting nucleic acid residues in a gene encoding the protein. The foregoing guidelines for mutation of Ubc domains may be applied to UBDs.

Zinc finger UBOs are known, for instance, in HDAC6, where the UBD is located between residues 1133 and 1204 (SEQ ID No. 14); in RABEX5, wherein A20-like ZnF and MIU UBDs are located between amino acids 1 and 74 (SEQ ID No. 15); in NPL4, where it is between positions 104 and 246 (SEQ ID No. 16); in TAB2, where it lies between residues 663 and 693 (SEQ ID No. 17); and in IsoT, where it lies between residues 173 and 289 (SEQ ID No 18).

When selecting a UBD for fusing to an Ubc, it is preferred that the lysine specificity of the Ubc should be compatible with the binding of the UBD to ubiquitin. For example, if Lys-11 is the preferred linkage residue of the Ubc, the UBO preferably binds ubiquitin in such a manner as to leave Lys-11 accessible for chain extension with ubiquitin molecules.

A chimeric protein in accordance with the invention may comprise a Ubc domain fused to a UBO. The UBO is preferably C-terminal in the fusion, although N-terminal fusions are contemplated. Fusions may be created by covalent linkage of polypeptide domains, or ligation of nucleic acids encoding such domains in the form of restriction fragments, amplification fragments or both. Moreover, synthetic nucleic acids may be used to create synthetic or partially synthetic nucleic acids encoding a fusion protein in accordance with the invention.

Fusions useful in the present invention include UBE2S and UBE2C fusions, for the production of Lys-11 linked polyubiquitin. The Ubc domains of UBE2S and UBE2C may be ligated to UBDs from a variety of proteins. For example, zinc finger UBDs may be used, such as the domains derived from polymerase-h or polymerase-k, Tax1BP1, NPL4, Vps63, TAB2, TA83, RABEXS, A20, IsopeptidaseT (IsoT) and HDAC6.

Preferred combinations include the Ube of UBE2S and the ZnF UBP domain of IsopeptidaseT, as well as the Ubc of UBE2S and the NZF of TAB2.

For example, the engineered UBE2S-UBD fusion protein is constructed making use of a naturally occurring Ncol restriction site in the human UBE2S sequence just before the Lys-rich tail (residue 196), and cloned into a vector such as pGEX6P1(Amersham). The IsoT(USP5) ZnF UBP domain (residues 173-289) are amplified from cDNA with primers

UBP-FW [SEQ ID No 21] 5′-CCAAGGTT CCATGG TACGGCAGGTGTCTAAGCATGCC-3′ and UBP-RV [SEQ ID No 22] 5′-GCCTA GCGGCCGC TTATGTCTTCTGCATCTTCAGCAT-  GTCGATG-3′). The amplified fragment is ligated into the Ncol/Notl restriction sites present in the pGEX6P1-UBE2S expression plasmid. The protein is expressed in E. coli and purified.

The TAB2 NZF domain (Amino Acids 663-693; Nucleotides 1988-2079+STOP) is amplified using primers NZFfus663FW: CCAAGGTTCCATGGATGAGGGAGCTC-AGTGGAATTG [SEQ ID No 23] and NZFfus693RV: GCCTAGCGGCCGCTTATC-AGAAATGCCTTGGCATCTC [SEQ ID No 24]. As with the ZnF domain, the amplified fragment is restriction digested and ligated into the Ncol/Notl restriction sites present in the pGEX6P1-UBE2S expression plasmid.

A similar technique may be employed for making Ubc-UBD fusions of choice.

A wide variety of expression systems are available for the production of chimeric polypeptides. For example, expression systems of both prokaryotic and eukaryotic origin may be used for the production of E2 fusion proteins.

Nucleic acid vectors are commonly used for protein expression. The term “vector” refers to a nucleic acid molecule that may be used to transport a second nucleic acid molecule into a cell, and/or express it therein. In one embodiment, the vector allows for replication of DNA sequences inserted into the vector. The vector may comprise a promoter to enhance expression of the nucleic acid molecule in at least some host cells. Vectors may replicate autonomously (extrachromosomal) or may be integrated into a host cell chromosome. In one embodiment, the vector may comprise an expression vector capable of producing a fusion protein derived from at least part of a nucleic acid sequence inserted into the vector.

A cloning vector may be a nucleic acid molecule, such as a plasmid, cosmid, or bacteriophage, that has the capability of replicating autonomously in a host cell. Cloning vectors typically contain one or a small number of restriction endonuclease recognition sites that allow insertion of a nucleic acid molecule in a determinable fashion without loss of an essential biological function of the vector, as well as nucleotide sequences encoding a marker gene that is suitable for use in the identification and selection of cells transformed with the cloning vector. Marker genes typically include genes that provide tetracycline resistance or ampicillin resistance.

An expression vector typically may comprise a transcription promoter, a gene, and a transcription terminator. Expression vectors may be autonomously replicating, or integrated into the host genome. Gene expression is usually placed under the control of a promoter, and such a gene is said to be operably linked to the promoter. Similarly, a regulatory element and a core promoter are operably linked if the regulatory element modulates the activity of the core promoter. The nucleic acid encoding the chimeric enzyme according to the invention is typically expressed under the control of a promoter in an expression vector.

To express a gene, a nucleic acid molecule encoding the protein must be operably linked to regulatory sequences that control transcriptional expression and then, introduced into a host cell. In addition to transcriptional regulatory sequences, such as promoters and enhancers, expression vectors may include transcriptional and translational regulatory sequences. The sequences used will be appropriate to the host, which may be prokaryotic or eukaryotic. The transcriptional and translational regulatory signals suitable for a mammalian host may be derived from viral sources, such as adenovirus, bovine papilloma virus, simian virus, or the like, in which the regulatory signals are associated with a particular gene that has a high level of expression. Suitable transcriptional and translational regulatory sequences also may be obtained from mammalian genes, such as actin, collagen, myosin, and metallothionein genes. Prokaryotic regulatory sequences may similarly be derived from viral genes, and are known in the art.

The inclusion of an affinity tag is useful for the identification or selection of cells expressing the fusion protein. Examples of affinity tags include polyHistidine tags (which have an affinity for nickel-chelating resin), c-myc tags, which are detected with anti-myc antibodies, calmodulin binding protein (isolated with calmodulin affinity chromatography), substance P, the RYIRS tag (which binds with anti-RYIRS antibodies), a hemagglutinin A epitope tag, which is detected with an antibody, the Glu-Glu tag, and the FLAG tag (which binds with anti-FLAG antibodies). Nucleic acid molecules encoding such peptide tags are available, for example, from Sigma-Aldrich Corporation (St. Louis, Mo., USA).

The gram-negative bacterium E. coli is widely used as a host for heterologous gene expression. Although large amounts of heterologous protein may accumulate inside the cell, this expression system is effective in the context of the present invention. Suitable strains of E. coli include BL21(DE3), BL21(DE3)pLysS, BL21(DE3)pLysE, DH1, DH41, DH5, DH51, DH51F′, DH51MCR, DH10B, DH10B/p3, DH11S, C600, HB101, JM101, JM105, JM109, JM110, K38, RR1, Y1088, Y1089, CSH18, ER1451, and ER1647.

Bacteria from the genus Bacillus are also suitable as heterologous hosts, and have capability to secrete proteins into the culture medium. Other bacteria suitable as hosts are those from the genera Streptomyces and Pseudomonas. Suitable strains of Bacillus subtilus include BR151, YB886, Ml119, Ml120, and B170 (see, for example, Hardy, “Bacillus Cloning Methods,” in DNA Cloning: A Practical Approach, Glover (ed.) (IRL Press 1985)). Standard techniques for propagating vectors in prokaryotic hosts are well-known to those of skill in the art (see, for example, Ausubel 1995; Wu et al., Methods in Gene Biotechnology (CRC Press, Inc. 1997)).

Eukaryotic hosts such as yeasts or other fungi may be used. In general, yeast cells are preferred over fungal cells because they are easier to manipulate. However, some proteins are either poorly secreted from the yeast cell, or in some cases are not processed properly (e.g. hyperglycosylation in yeast). In these instances, a different fungal host organism should be selected.

The use of suitable eukaryotic host cells—such as yeast, fungal and plant host cells—may provide for post-translational modifications (e.g. myristoylation, glycosylation, truncation, lapidation and tyrosine, serine or threonine phosphorylation) as may be needed to confer optimal biological activity on recombinant expression products.

In some embodiments, the fusion proteins may be expressed as GST fusions. For example, the pGEX vector system employs a GST fusion. Use of GST as a fusion partner provides an inducible expressions system which facilitates the production of proteins in the E. coli system. Proteins expressed using this system may be isolated using a glutathione capture resin.

For example, recombinant GST-UBE2S constructs are expressed in Rosetta 2 (DE3) placI eels (Novagen). 1 L cultures of cells are induced at OD₆₀₀ of 0.6 with 250 μM IPTG and proteins are expressed at 20° C. overnight. Cells are harvested and flash-frozen. 30 ml lysis buffer containing 270 mM sucrose, 50 mM Tris (pH 8.0), 50 mM NaF, 1 protease inhibitor cocktail tablet (Roche) (0.1% v/v β-mercaptoethanol, 1 mg/ml lysozyme and 0.1 mg/ml DNase) are added per liter of culture. After sonication, cell lysates are cleared using a Sorvall SS-34 rotor (18,000 rpm, 30 min, 4° C.) and supernatants are incubated with Glutathione Sepharose 4B (GE Healthcare) for 1 h to immobilize soluble GST fusion proteins. Subsequently, the sepharose beads are washed with 500 ml high salt buffer [500 mM NaCl, 25 mM Tris (pH 8.5), 5 mM DTT] and 300 ml low salt buffer [150 mM NaCl, 25 mM Tris (pH 8.5), 5 mM DTT]. For site-specific cleavage of the GST tag, immobilized fusion proteins are incubated with 30 mM PreScission protease (GE Healthcare) overnight. Cleaved proteins are eluted with low salt buffer and flash-frozen in liquid nitrogen. All samples are >95% pure after purification.

The chimeric E2 enzymes of the invention produce enhanced levels of free polyubiquitin, compared to naturally occurring E2. Assays for ubiquitination are known in the art; for instance, a description of such assays, and relevant background, is set forth, for example, in WO2009134897, US2006088901 and WO2004020674. Ubiquitination assays kits are available commercially, for instance from Cisbio, Bedford, Mass., USA; Invitrogen, Carlsbad, Calif., USA; and Enzo Lifesciences, Plymouth Meeting, Pa., USA.

In general, an assay for the production of free ubiquitin requires the incubation of E1 enzyme, the chimeric E2 according to the invention and monomeric ubiquitin in the presence of ATP in a buffer solution.

E1 enzymes are available commercially, for instance from Enzo Lifesciences. A list of E1 enzymes is set forth in Table 1 of WO2004020674.

In one embodiment, ubiquitin may be labelled, to facilitate its subsequent detection or isolation.

In one embodiment, 30 μl reactions may be carried out at 37° C. containing 25 ng ubiquitin-activating enzyme (E1), 2 μg ubiquitin conjugating enzyme (E2), 5 μg ubiquitin, 10 mM ATP, 40 mM Tris (pH 7.5), 10 mM MgCl₂ and 0.6 mM DTT. After 1 h the reaction is stopped by addition of 10 μl 4×LDS sample buffer (Invitrogen), resolved by SDS-PAGE on 4-12% precast gels and subjected to Western analysis using rabbit polyclonal anti-ubiquitin antibody (Upstate).

The scale of the reactions may be increased, if necessary. Performing the reaction with naturally occurring E2S does not result in the formation of significant amounts of polyubiquitin. However, using a chimeric E2 according to the invention, polyubiquitin chains may be isolated and purified.

In one embodiment, ubiquitin dimers are synthesized by incubating 16 μg E1 enzyme, 100 μg UBE2SΔC, 12.5 mg ubiquitin, 10 mM ATP, 40 mM Tris (pH 7.5), 10 mM MgCl₂ and 0.6 mM DTT at 37° C. overnight. Subsequently, 50 mM DTT is added to the reaction before further dilution with 14 ml of 50 mM ammonium acetate (pH 4.5).

K11-linked diubiquitin may be purified by cation exchange using a MonoS column (GE Healthcare) and concentrated to 5 mg/ml. Crystals are formed after 1 day from 3 M NaCl and 0.1 M citric acid (pH 3.5). Crystals may be soaked in mother liquor containing 15% ethylene glycol before freezing in liquid nitrogen.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined in the appended claims.

The present invention will be further illustrated in the following Examples which are given for illustration purposes only and are not intended to limit the invention in any way.

EXAMPLES Example 1 Analysis of E2 Enzymes Involved in K11 Chain Formation

For the assembly of free K48- and K63-linked ubiquitin chains, specific E2 enzymes have been described, and the biology of these posttranslational modifications is now known in great detail. In order to study the elusive K11 linkage, Applicants analyzed the in vitro properties of two human E2 conjugating enzymes that have been associated with this chain type, namely UBE2C/UbcH10 (Jin et al., 2008) and UBE2S/E2-EPF (Baboshina & Haas, 1996). Applicants tested whether UBE2S and UBE2C would assemble unattached polyubiquitin chains in vitro in absence of an E3 ligase. Analytical assays were carried out in 30 μl reactions at 37° C. containing 250 nM ubiquitin-activating enzyme (E1), 2.8 μM (UBE2S) or 3.4 μM (UBE2C) ubiquitin conjugating enzyme (E2), 19.5 μM ubiquitin, 10 mM ATP, 40 mM Tris (pH 7.5), 10 mM MgCl₂ and 0.6 mM OTT. After 1 h the reaction was stopped by addition of 10 μl 4×LDS sample buffer (Invitrogen), resolved by SDS-PAGE on 4-12% precast gels and subjected to Western analysis using rabbit polyclonal anti-ubiquitin antibody (Upstate). Applicants found that UBE2S generated small amounts of free diubiquitin, as judged by the appearance of ubiquitin dimers on reducing SDS PAGE gels (FIG. 1 a), while UBE2C did not assemble unattached ubiquitin chains (FIG. 1 b). UBE2S, but not UBE2C, also underwent autoubiquitination, resulting in the appearance of high molecular weight species of UBE2S (FIG. 1 a). Linkage type analysis using single-Lys ubiquitin mutants (K6-only, K11-only etc.) revealed that UBE2S assembled K11 linkages specifically (FIG. 1 a), since ubiquitin dimers as well as high molecular weight forms of UBE2S were only observed with the K11-only ubiquitin mutant (FIG. 1 a). UBE2S autoubiquitinated several of its 17 Lys residues, however with ubiquitin mutants lacking K11, these monoubiquitin modifications were not extended (FIG. 1 a, c), and autoubiquitination with Lysless (K0) and K63-only ubiquitin followed similar kinetics resulting in 6-7 distinct multi-monoubiquitinated bands of UBE2S (FIG. 1 c). To verify that UBE2S was K11-specific also with wild-type ubiquitin, Applicants performed LC-MS/MS analysis of trypsinized diubiquitin.

LC-MS/MS was carried out by nanoflow reverse phase liquid chromatography (using a U3000 from Dionex) coupled online to a Linear Ion Trap (LTQ)-Orbitrap XL mass spectrometer (Thermo Scientific). Briefly, the LC separation was performed using a C18 PepMap capillary column (75 μm ID×150 mm; Dionex) and the peptides were eluted using a linear gradient from 5% B to 50% B over 40 minutes at a flow rate of 200 nL/min (solvent A: 98% H₂O; 2% acetonitrile in 0.1% formic acid; solvent B: 90% acetonitrile in 0.1% formic acid). The eluted peptides were electrosprayed into the mass spectrometer via a nanoelectrospray source fitted with a PicoTip emitter (New Objective). A cycle of one full FT scan mass spectrum (350-2000 m/z, resolution of 60 000 at m/z 400) was followed by 6 data-dependent MS/MS acquired in the linear ion trap with normalized collision energy (setting of 35%). Target ions already selected for MS/MS were dynamically excluded for 60 s. Peptides were identified from MS/MS spectra by searching against a Swissprot database using the Mascot search algorithm (matrixscience.com) and Proteome Discoverer (Thermo Fisher Scientific). Oxidation of methionine, GlyGly and LeuArgGlyGly addition on Lysine residues were used as variable modifications. Initial mass tolerance was set to 10 ppm for peptide parent mass, 0.8 Da for fragment masses and enzyme restriction was set to trypsin specificity with 2 missed cleavages.

Applicants detected peptides derived from K11-linked diubiquitin, and with significantly less intensity also from K48- and K63-linked diubiquitin. Other linkages were not detected. Applicants focused on UBE2S and set out to harness its capability to produce free K11-linked ubiquitin chains.

Human UBE2S may comprise 222 residues with an N-terminal conserved catalytic Ubc domain spanning residues 1-156. The very C-terminal 25 residues of UBE2S encompass nine Lys residues that are conserved in UBE2S homologs (ensembl.org), while the remaining 40 residues form a non-conserved Lys-free linker (FIG. 2 a). Mutation of the catalytic Cys residue in the Ubc domain to Ala (UBE2S^(C95A)) rendered UBE2S inactive, while mutation to Ser (UBE2S^(C95S)) acted as a ubiquitin-trapping mutant, in which the Ser residue was still charged with ubiquitin by the E1 enzyme, but failed to discharge efficiently, similar to what has been reported for UBE2N/Ubc 1317 (FIG. 2 a).

Autoubiquitination of UBE2S occurred in cis, as wild-type UBE2S was unable to ubiquitinate GST-tagged UBE2S^(C95A) in trans, despite being able to autoubiquitinate itself (FIG. 2 b). The autoubiquitination of UBE2S appeared to be favored compared to formation of free K11-linked chains, and free chain production is inefficient. The Lys-rich tail of UBE2S is a likely target for autoubiquitination. Removal of the last 25 residues (UBE2SΔC) reduced autoubiquitination (FIG. 2 c), increased formation of free diubiquitin (data not shown), and the enzyme remained specific for K11 linkages (FIG. 2 c). From 25 mg input ubiquitin, ˜1 mg K11-linked diubiquitin could be purified by cation exchange (FIG. 2 d).

Example 2 Generation of K11-Linked Ubiquitin Tetramers

In order to increase the yields of K11-linked dimers and to obtain longer polymers, Applicants reverted to protein engineering to create an UBE2S variant with increased capability to form free ubiquitin chains. Having established that the Lys-rich tail of UBE2S is polyubiquitinated by UBE2S in a cis reaction, Applicants replaced this tail (residues 196-222) with the ubiquitin binding ZnF-UBP domain of human USP5/IsoT (residues 173-289) (Reyes-Turcu et al., 2006; FIG. 3 a). This UBD has two advantageous features: it binds ubiquitin with nanomolar affinity, and interacts with the free C-terminal tail of ubiquitin leaving the Lys11 side chain accessible for chain elongation. The UBE2S-UBD fusion protein was significantly more efficient in producing ubiquitin dimers, trimers, and tetramers.

Ubiquitin tetramers were synthesized by incubating 250 nM E1 enzyme, 4.8 μM UBE2S-UBD, 2.9 mM ubiquitin, 400 nM AMSH, 10 mM ATP, 40 mM Tris (pH 7.5), 10 mM MgCl₂ and 0.6 mM DTT in a 1 ml reaction at 37° C. After 1.5 hours 400 nM AMSH was added again to counteract the formation of K63-linked ubiquitin chains. After 3 hours, 50 mM DTT was added to the reaction before further dilution with 14 ml of 50 mM ammonium acetate (pH 4.5). K11-linked di-, tri- and tetraubiquitin were purified by cation exchange using a MonoS column (GE Healthcare) (FIG. 3). It was also possible to use K11-linked diubiquitin as input material to obtain tetraubiquitin.

Specificity analysis showed that UBE2S-UBD also incorporated K63-linkages in these oligomers (see K63-only mutant in FIG. 3 a). Two distinct trimer bands were observed in reactions using wild-type ubiquitin, but not with the K11-only ubiquitin mutant, indicating alternating or branched linkages with wild-type ubiquitin, since differently linked ubiquitin chains have distinct electrophoretic mobility (FIG. 3 a, b). Two linkage types (K11 and K63) in the wild-type ubiquitin reaction were further confirmed by LC-MS/MS analysis.

Formation of K63-linkages could be counteracted by either using the ubiquitin K63R mutant, or by incubation with the K63-specific deubiquitinase AMSH (McCullough et al., 2004; FIG. 3 b). Indeed, AMSH removed only the faster migrating of the two triubiquitin bands, showing that a chain with alternate linkages had been created by UBE2S-UBD (FIG. 3 b). When Applicants included AMSH directly in the assembly reactions, Applicants were able to remove the contaminating K63 linkages in situ (FIG. 3 b). This protocol allowed large scale generation and purification of K11-linked di-, tri- and tetraubiquitin (FIG. 3 c, d, e) with improved yields. Almost 50% of the input ubiquitin was converted into K11-linked oligomers using UBE2S-UBD, while UBE2SΔC only assembled 15% of input ubiquitin into K11-linked dimers (FIG. 3 b, compare integrated peak area in FIGS. 2 d and 3 d).

Example 3 Structure of K11-Linked Polyubiquitin

Generation of K11-linked ubiquitin chains in large quantities allowed detailed structural analysis of this chain type.

Large-scale ubiquitin chain assembly was carried out in 1 ml reactions. Ubiquitin dimers were synthesized by incubating 250 nM E1 enzyme, 4.8 μM UBE2SΔC, 1.5 mM ubiquitin, 10 mM ATP, 40 mM Tris (pH 7.5), 10 mM MgCl₂ and 0.6 mM DTT at 37° C. overnight. Subsequently, 50 mM DTT was added to the reaction before further dilution with 14 ml of 50 mM ammonium acetate (pH 4.5) to precipitate enzymes. The solution was filtered through a 0.2 μm syringe filter and K11-linked diubiquitin was purified by cation exchange using a MonoS column (GE Healthcare) and concentrated to 5 mg/ml. Crystals formed after 1 day from 3M NaCl and 0.1 M citric acid (pH 3.5). Before freezing in liquid nitrogen, crystals were soaked in mother liquor containing 15% ethylene glycol.

Diffraction data on crystals of K11-linked diubiquitin were collected on ESRF beamline ID14-EH2 (Grenoble). The crystals diffracted to a maximum resolution of 2.2 Å and displayed an orthorhombic space group that Pointless (Evans et al., 2006) identified to be most likely P222₁. The structure was solved by molecular replacement in MolRep (Vagin & Teplyakov, 2000), which identified 12 ubiquitin molecules from using monoubiquitin as a search model. The 12 molecules were related by translational symmetry, and formed two equivalent tetraubiquitin complexes with linkage ambiguity (FIG. 5), and another two diubiquitin molecules in which a two-fold axis generated the remaining dimers to form similar tetrameric assemblies. The structure was built in coot (Emsley & Cowtan, 2004) from the molecular replacement model, and refined in Phenix (Adams et al., 2002) using NCS, simulated annealing (initially) and TLS B-factor refinement at later stages of the refinement. Restraints for the isopeptide linkage were generated using phenix.elbow. Data collection and refinement statistics may be found in Table 2.

TABLE 2 Data collection and refinement statistics K11-linked diubiqulin Data collection Space group P222₁ Cell dimensions a, b, c (Å) 79.23, 79.96, 221.23 abc (°) 90, 90, 90 Resolution (Å) 24.92-2.20 (2.32-2.20)* R_(sym) or R_(merge) 0.106 (0.489) //s/ 6.0 (2.0) Completeness (%) 98.3 (99.7) Redundancy 3.1 (3.0) Refinement Resolution (Å) 24.92-2.20  No. reflections 65986 R_(work)/R_(free) 0.205/0.252 No. atoms Protein 7255 (12 ubiquitin molecules) Ligandion 111 Water 654 B-factors Protein 30.1 Ligandion 41.8 Water 34.4 R.m.s. deviations Bond lengths (Å) 0.005 Bond angles (°) 0.978 *Values in parentheses are for highest-resolution shell.

K11-linked diubiquitin adopts a compact conformation distinct from any other ubiquitin chain structure observed to date (FIG. 4 a). Contacts between ubiquitin moieties are entirely polar and do not involve the hydrophobic ubiquitin surface patch (Ile44, Leu8, Val70), which is the most common ubiquitin interaction site (FIG. 4 b). The interface instead forms between a surface centered on Glu24 of the distal ubiquitin, and a surface around Lys29 and Lys33 of the proximal ubiquitin. Several direct and water-mediated interactions are formed, including Arg74^(dist)-Glu34^(prox, backbone (bb)), Arg72^(dist)-Glu34^(prox, bb), Asp39^(dist bb)-Asp32^(prox, bb) (FIG. 4 c). The crystal structure was obtained at pH 3.5 in presence of 3 M sodium chloride. These conditions may mask additional charged interactions, for example Lys33^(prox)-Asp52^(dist), which are in close proximity but do not seem to interact in any of the dimer interfaces.

A striking feature of the crystal structure is the exposed location of the ubiquitin hydrophobic patch (FIG. 4 b). In the crystal, eight of the twelve hydrophobic patches are not involved in crystal contacts but point towards solvent channels. Furthermore, the hydrophobic patch is extended by Leu71 and Leu73 from the C-terminal tail of ubiquitin (FIG. 4 d). Since Arg72 and Arg74 are integral residues of the polar K11-diubiquitin interface, Leu71/Leu73 point outwards and are restrained unlike in monoubiquitin where the C-terminal tail is more mobile. Leu71/Leu73 therefore effectively increase the hydrophobic Ile44-surface (FIG. 4 c, d). With this larger hydrophobic patch, interaction of K11-linked chains with UBDs is likely to result in new interaction modes. In particular, proteins with tandem UBDs may be well suited to interact with adjacent hydrophobic patches in K11-linked polyubiquitin. Alternatively, novel classes of UBDs may recognize the unique structural features of K11-linked chains.

Ubiquitin chains are dynamic entities and may adopt multiple conformations in solution. The solution properties of K11-linked ubiquitin chains were studied with Nuclear Magnetic Resonance (NMR) spectroscopy.

¹³C, ¹⁵N-labeled ubiquitin K63R or K11R mutant was expressed from a pET17b plasmid in Rosetta2 (DE3) pLac1 cells. A 100 ml overnight culture grown in LB medium was pelleted and resuspended in modified K-MOPS minimal media (Neidhardt et al., 1974), lacking nitrogen and carbon sources. This was used to inoculate 3 L modified K-MOPS media supplemented with ¹³C glucose/¹⁵N ammonium chloride. Protein expression was induced after 16 hrs growth at 30° C. with 0.4 mM IPTG, and cells were harvested after a further 4 hrs. Mutant ubiquitin was purified according to Pickart & Rassi, 2005. To obtain only distally labeled K11-linked diubiquitin, wild-type ubiquitin was mixed with ¹³C, ¹⁵N-labeled ubiquitin K11R mutant in a 1:2 ration in a chain assembly reaction. Prior to data acquisition, samples were dialyzed either against phosphate buffered saline (150 mM NaCl, 18 mM Na2HPO₄, 7 mM NaH₂PO₄×2H₂O, pH 7.4) or against 150 mM NaCl, 50 mM NH₄Ac (pH 3.5) in 3 kDa cut-off Slide-A-Lyzer dialysis cassettes (Thermo Scientific).

NMR experiments were acquired on Bruker DRX600 MHz and AV2+ 700 MHz spectrometers equipped with cryogenic triple resonance TCI probes and at a temperature of 298K; all data were processed in Topspin 2.1 (Bruker, Karlsruhe) and analyzed in Sparky (Goddard & Kneller, UCSF). Weighted chemical shift perturbations were measured in ¹⁵N fast HSQC experiments (Mori et al., 1995) and defined as ((D¹H)²)^(0.5)+((D¹⁵N/5)²)^(0.5) [ppm](Hadjuk et al., 1997). Standard triple resonance experiments (HNCACB, CBCA(CO)NH and HNCA) were used to assign all mono- and di-ubiquitin K63R or K11R species and confirm the identity of shifted and doubled resonances.

¹H, ¹⁵N-heteronuclear correlation spectra (HSQC) provide a fingerprint of the local environment of individual residues. These so-called chemical shifts report on the resonance frequencies of all backbone amide protons and nitrogens, and chemical shift perturbations as a consequence of e.g. the formation of an interface are highly specific.

Applicants assembled uniformly labeled K11 diubiquitin from ¹³C, ¹⁵N-labeled K63R mutant ubiquitin. To subsequently deconvolute the contribution from both parts of the interface, in a second species only the distal moiety of K11-linked diubiquitin was ¹³C, ¹⁵N-labeled. To achieve this, assembly reactions with ¹³C, ¹⁵N-labeled K11R and unlabeled wild-type ubiquitin were performed, in which the K11R mutant serves as a distal chain terminator. To minimize buffer effects, the two labelled diubiquitin species, as well as labelled K63R, and K11R monoubiquitin (all at 100 μM) were dialyzed simultaneously against neutral (pH 7.4) or acidic (pH 3.5) buffer also containing 150 mM NaCl to mask nonspecific interactions. Relaxation experiments and measurements at different concentrations confirmed monodispersity, and allowed to exclude aggregation effects for all species at the chosen experimental conditions. Applicants assigned and confirmed the chemical shift positions in all species with standard tripleresonance experiments (Supp. FIG. 4 a). To generate chemical shift perturbation maps, Applicants compared uniformly labeled K11-linked diubiquitin to K63R monoubiquitin, and distally labeled K11-linked diubiquitin to K I R monoubiquitin. To assess the effect of K63R and K11R mutations Applicants compared the labeled monoubiquitin species to find perturbation differences of <0.1 ppm, with exception of the flexible loop region in ubiquitin near K11 that is slightly more perturbed.

Immediately apparent was the doubling of a defined subset of resonances in the spectra of uniformly labeled K11-linked diubiquitin, associated with the formation of a non-symmetric interface (FIG. 5 a). As expected, the resonances for Lys11 and Gly76 involved in the K11-linkage were significantly shifted compared to monoubiquitin (FIG. 5 b). The chemical shift perturbation map of this species contains contributions of both sides of the interface (FIG. 5 b). To deconvolute the individual contributions, Applicants analyzed chemical shift perturbations of distally labeled K11-linked diubiquitin in comparison to K11R monoubiquitin (FIG. 5 b). This revealed the set of perturbed resonances that correspond to the interface on the distal moiety. Importantly, all resonances that were found to be perturbed in the distally labelled K11-linked diubiquitin have equivalent or similar perturbations in the uniformly labelled K11-linked diubiquitin. However, Applicants cannot exclude or quantify contributions to these perturbations from the proximal moiety in this case.

This analysis shows that K11-linked dimers have a defined pattern of perturbed resonances in solution, which is distinct from the pattern observed for K48-, or K63-linked diubiquitin (Varadan et al., 2004; Varadan et al., 2002; Tenno et al., 2004; FIG. 5 d), reflecting (a) unique conformation(s) of K11-linked diubiquitin. Consistent with the crystallographic analysis, the backbone resonances corresponding to residues 41-51 of ubiquitin (including Ile44) are not perturbed, suggesting that this region which is involved in the K48 diubiquitin interface (Varadan et al., 2002; Tenno et al., 2004; FIG. 5 d) and in most ubiquitin-UBD interactions (Zhang et al., 2009; Varadan et al., 2005; Raasi et al, 2005), is not involved in the dimer interface in K11-linkages. Instead, the chemical shift perturbations indicate three regions of the ubiquitin surface that contribute to the interface and/or are affected by the K11 isopeptide bond: The flexible β-hairpin loop spanning residues 5-15, possibly as a consequence of the isopeptide bond at K11; residues 29-36 that include the C-terminal part of the ubiquitin helix; and the C-terminal residues from 69-76 (FIG. 5 b). Mapping of these residues onto the surface of ubiquitin reveals that the perturbed resonances correspond to a surface that is almost identical to the proximal interaction interface observed in the crystal structure (FIG. 5 e). A corresponding distal interface however appears to be more distinct when compared to the crystal structure (FIG. 5 f). At this interface, two residues, Gly53 and Asp24, remain exchange broadened as in monomeric ubiquitin (white in FIG. 5 f), indicating that this region of the interface is dynamic and may adopt multiple conformations. Similar observations of exchange broadening in interface residues have been made in the case of K48-linked diubiquitin molecules (Varadan et al., 2002). However, two further residues that reside on the distal interface of the crystal structure, Asp39 and 10 Asp52 are also unperturbed (circled in FIG. 5 f), indicating that in solution, the distal ubiquitin may rotate or move slightly, adjusting the interface.

To further distinguish between interface residues and residues perturbed as a result of forming the isopeptide linkage, Applicants analyzed chemical shift perturbations also at low pH. It has previously been shown for K48-linked diubiquitin that low pH ‘opens’ the compact conformation of this chain type resulting in a more transient interface21. If a similar interface ‘opening’ also occurred for K11-linkages, this may allow to define interface residues more confidently. Although at pH 3.5 Applicants observe fewer perturbations compared to pH 7.4, several residues remain perturbed (FIG. 5 c). On the other hand, K29 and K33 are perturbed only at pH 7.4 but do not show significant perturbation at pH 3.5 (indicated by arrows in FIG. 5 b,c). This suggests that these residues are located at an interface at pH 7.4.

In summary, the crystal structure represents most likely a more compact conformation compared to the conformation(s) of K11-linked diubiquitin in solution. However, solution studies also reveal significant perturbations indicative of an interface and hence a compact conformation of K11-linked diubiquitin. The distinct perturbation pattern suggests that K29 and K33 reside at the diubiquitin interface, which would result in a unique conformation compared to K48- and K63-linked diubiquitin. The data also highlight the dynamic nature of K11-linked ubiquitin chains. Further analysis will be required to analyze preferred domain orientations in K11-linked ubiquitin chains in solution. Taken together, the unique structural features of K11-linked diubiquitin highlight the conformational variability of differently linked ubiquitin chains (FIG. 5 b,d).

REFERENCES

-   Komander, D. The emerging complexity of protein ubiquitination.     Biochem Soc Trans 37. 937-53 (2009). -   Chen, Z. J. & Sun, L. J. Nonproteolytic functions of ubiquitin in     cell signaling. Mol Cell 33, 275-86 (2009). -   Hershko, A. & Ciechanover, A. The ubiquitin system. Annu Rev Biochem     67, 425-79 (1998). -   Xu, P. et al. Quantitative proteomics reveals the function of     unconventional ubiquitin chains in proteasomal degradation. Cell     137, 133-45 (2009). -   Peng, J. et al. A proteomics approach to understanding protein     ubiquitination. Nat Biotechnol 21, 921-6 (2003). -   Dye, B. T. & Schulman, B. A. Structural mechanisms underlying     posttranslational modification by ubiquitin-like proteins. Annu Rev     Biophys Biomol Struct 36, 131-50 (2007). -   Ye, Y. & Rape, M. Building ubiquitin chains: E2 enzymes at work. Nat     Rev Mol Cell Biol 10, 755-64 (2009). -   Hofmann, R. M. & Pickart, C. M. Noncanonical MMS2-encoded     ubiquitin-conjugating enzyme functions in assembly of novel     polyubiquitin chains for DNA repair. Cell 96, 645-53 (1999). -   Chen, Z. & Pickart, C. M. A 25-kilodalton ubiquitin carrier protein     (E2) catalyzes multi-ubiquitin chain synthesis via lysine 48 of     ubiquitin. J Biol Chem 265, 21835-42 (1990). -   Baboshina, O. V. & Haas, A. L. Novel multiubiquitin chain linkages     catalyzed by the conjugating enzymes E2EPF and RAD6 are recognized     by 26 S proteasome subunit 5. J Biol Chem 271, 2823-31 (1996). -   Jin, L., Williamson, A., Banerjee, s., Philipp, I. & Rape, M.     Mechanism of ubiquitin-chain formation by the human     anaphase-promoting complex. Cell 133, 653-65 (2008). -   Alexandru, G. et al. UBXD7 binds multiple ubiquitin ligases and     implicates p97 in HIF1 alpha turnover. Cell 134, 804-16 (2008). -   Reyes-Turcu, F. E. et al. The ubiquitin binding domain ZnF UBP     recognizes the C-terminal diglycine motif of unanchored ubiquitin.     Cell 124, 1197-208 (2006). -   McCullough, J., Clague, M. J. & Urbe, S. AMSH is an     endosome-associated ubiquitin isopeptidase. J Cell Biol 166, 487-92     (2004). -   Varadan, R. et al. Solution conformation of Lys63-linked     di-ubiquitin chain provides clues to functional diversity of     polyubiquitin signaling. J Biol Chem 279, 7055-63 (2004). -   Varadan, R., Walker, O., Pickart, C. & Fushman, D. Structural     properties of polyubiquitin chains in solution. J Mol Biol 324,     637-47 (2002). -   Tenno, T. et al. Structural basis for distinct roles of Lys63- and     Lys48-linked polyubiquitin chains. Genes Cells 9, 865-75 (2004). -   Zhang, N. et al. Structure of the s5a:k48-linked diubiquitin complex     and its interactions with rpn13. Mol Cell 35, 280-90 (2009). -   Varadan, R., Assfalg, M., Raasi, S., Pickart, C. & Fushman, D.     Structural determinants for selective recognition of a Lys48-linked     polyubiquitin chain by a UBA domain. Mol Cell 18, 687-98 (2005). -   Raasi, S., Varadan, R., Fushman, D. & Pickart, C. M. Diverse     polyubiquitin interaction properties of ubiquitin-associated     domains. Nat Struct Mol Biol 12, 708-14 (2005). -   Evans, P. Scaling and assessment of data quality. Acta Crystalogr D     Biol Crystallogr 62, 72-82 (2006). -   Vagin, A. & Teplyakov, A. An approach to multi-copy search in     molecular replacement. Acta Crystallogr D Biol Crystallogr 56,     1622-4 (2000). -   Emsley, P. & Cowtan, K. Coot: model-building tools for molecular     graphics. Acta Crystallogr D Biol Crystallogr 60, 2126-32 (2004). -   Adams, P. D. et al. PHENIX: building new software for automated     crystallographic structure determination. Acta Crystallogr D Biol     Crystallogr 58, 1948-54 (2002). -   Neidhardt, F. C., Bloch, P. L. & Smith, D. F. Culture medium for     enterobacteria. J Bacteriol 119, 736-47 (1974). -   Pickart, C. M. & Raasi, S. Controlled synthesis of polyubiquitin     chains. Methods Enzymol 399, 21-36 (2005). -   Mori, S., Abeygunawardana, C. Johnson, M. O. & van Zijl, P. C.     Improved sensitivity of HSQC spectra of exchanging protons at short     interscan delays using a new fast HSQC (FHSQC) detection scheme that     avoids water saturation. J Magn Reson B 108, 94-8 (1995). -   Hajduk, P. J. et al. NMR-based discovery of lead inhibitors that     block DNA binding of the human papillomavirus E2 protein. J Med Chem     40, 3144-50 (1997). -   Dikic, et al., Ubiquitin-binding domains—from structures to     functions. Nat Rev Mol Biol 10:659-671, 2009 -   Hicke et al., Nat Rev Cell Biol 6:610-621, 2005

The invention is further described by the following numbered paragraphs:

1. An E2 enzyme comprising a Ubc domain, from which an N-terminal tail or a C-terminal tail has been removed.

2. An E2 enzyme according to paragraph 1, which is a chimeric enzyme wherein the Ubc is fused to a heterologous ubiquitin-binding domain (UBD).

3. A chimeric E2 enzyme according to paragraph 2, wherein the UBD is C-terminal to the Ubc domain.

4. A chimeric E2 enzyme according to paragraph 2 or paragraph 3, wherein the UBD is an α-helical, zinc finger or pleckstrin homology domain.

5. A chimeric E2 enzyme according to paragraph 2 or paragraph 3, wherein the UBD is a domain selected from the group consisting of UIM, IUIM (MIU), DUIM, UBM, UBA, GAT, CUE, VHS, UBZ, NZF, ZnF A20, ZnF UBP (PAZ), PRU, GLUE, UEV, UBC, SH3, PFU and Jab1/MNP domains.

6. A chimeric E2 enzyme according to paragraph 4 or paragraph 5, wherein the UBD is derived from Isopeptidase T.

7. A chimeric E2 enzyme according to paragraph 6, wherein the UBD comprises the sequence from about position 163 to about position 291 of Isopeptidase T.

8. A chimeric E2 enzyme according to paragraph 4 or paragraph 5, wherein the UBD is a UBA, UIM, ZnF or NZF domain.

9. An E2 enzyme according to any preceding paragraph, wherein the Ubc domain is derived from an E2 enzyme selected from the group consisting of UBE2A, UBE2B, UBE2C, UBE2D1, UBE2D2, UBE2D3, UBE2D4, UBE2E1, UBE2E2, UBE2E3, UBE2F, UBE2G1, UBE2G2, UBE2H, UBE21, UBE2J1, UBE2J2, UBE2K, UBE2L3, UBE2L6, UBE2M, UBE2N, UBE2NL, UBE2O, UBE2Q1, UBE2Q2, UBE2R1, UBE2R2, UBE2S, UBE2T, UBE2U, UBE2W, UBE2Z and BIRC6.

10. An E2 enzyme according to paragraph 9, wherein the E2 enzyme is a class II E2 enzyme.

11. An E2 enzyme according to paragraph 10, wherein an N-terminal or a C-terminal amino acid tail on the class II E2 enzyme is replaced by the UBD.

12. An E2 enzyme according to paragraph 10 or paragraph 11, wherein the Ubc domain is derived from UBE2S.

13. An E2 enzyme according to paragraph 12, wherein the Ubc domain comprises residues 1 to 156 of UBE2S.

14. A method for increasing the capacity of an E2 enzyme to produce free polyubiquitin chains in solution, comprising conjugating fusing the Ubc domain of said E2 enzyme to a UBD.

15. A method according to paragraph 14, wherein the E2 enzyme is selected from the group consisting of UBE2A, UBE2B, UBE2C, UBE2D1, UBE2D2, UBE2D3, UBE2D4, UBE2E1, UBE2E2, UBE2E3, UBE2F, UBE2G1, UBE2G2, UBE2H, UBE21, UBE2J1, UBE2J2, UBE2K, UBE2L3, UBE2L6, UBE2M, UBE2N, UBE2NL, UBE20, UBE2Q1, UBE2Q2, UBE2R1, UBE2R2, UBE2S, UBE2T, UBE2U, UBE2V1, UBE2V2, UBE2V3, UBE2W, UBE2Z, AKTIP and BIRC6 and the UBD is a domain selected from the group consisting of UIM, IUIM (MIU), DUIM, UBM, UBA, GAT, CUE, VHS, UBZ, NZF, A20-like ZnF, ZnF UBP (PAZ), PRU, GLUE, UEV, UBC, SH3, PFU and Jab1/MNP domains.

16. A method according to paragraph 15, wherein the E2 enzyme is UBE2S.

17. A method according to any one of paragraphs 14 to 16, wherein the UBD is a ZnF UBP domain.

18. A method for producing free polyubiquitin chains linked through a desired lysine residue, comprising the steps of: (a) selecting an E2 enzyme which possesses the desired lysine residue specificity; (b) fusing the Ubc catalytic domain of said E2 enzyme to a UBD ubiquitin binding domain; and incubating the resulting chimeric protein with an E1 ubiquitin activating enzyme and monomeric ubiquitin.

19. A method according to paragraph 18, wherein the incidence of undesired lysine linkages is reduced by including a linkage-specific deubiquitinase in the incubation.

Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the above paragraphs is not to be limited to particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope of the present invention. 

What is claimed is:
 1. An E2 enzyme comprising a Ubc domain, from which an N-terminal tail or a C-terminal tail has been removed.
 2. An E2 enzyme according to claim 1, which is a chimeric enzyme wherein the Ubc is fused to a heterologous ubiquitin-binding domain (UBD).
 3. A chimeric E2 enzyme according to claim 2, wherein the UBD is C-terminal to the Ubc domain.
 4. A chimeric E2 enzyme according to claim 2, wherein the UBD is an α-helical, zinc finger or pleckstrin homology domain.
 5. A chimeric E2 enzyme according to claim 2, wherein the UBD is a domain selected from the group consisting of UIM, IUIM (MIU), DUIM, UBM, UBA, GAT, CUE, VHS, UBZ, NZF, ZnF A20, ZnF UBP (PAZ), PRU, GLUE, UEV, UBC, SH3, PFU and Jab1/MNP domains.
 6. A chimeric E2 enzyme according to claim 4, wherein the UBD is derived from Isopeptidase T.
 7. A chimeric E2 enzyme according to claim 6, wherein the UBD comprises the sequence from about position 163 to about position 291 of Isopeptidase T.
 8. A chimeric E2 enzyme according to claim 4, wherein the UBD is a UBA, UIM, ZnF or NZF domain.
 9. An E2 enzyme according to claim 1, wherein the Ubc domain is derived from an E2 enzyme selected from the group consisting of UBE2A, UBE2B, UBE2C, UBE2D1, UBE2D2, UBE2D3, UBE2D4, UBE2E1, UBE2E2, UBE2E3, UBE2F, UBE2G1, UBE2G2, UBE2H, UBE2I, UBE2J1, UBE2J2, UBE2K, UBE2L3, UBE2L6, UBE2M, UBE2N, UBE2NL, UBE2O, UBE2Q1, UBE2Q2, UBE2R1, UBE2R2, UBE2S, UBE2T, UBE2U, UBE2W, UBE2Z and BIRC6.
 10. An E2 enzyme according to claim 9, wherein the E2 enzyme is a class II E2 enzyme.
 11. An E2 enzyme according to claim 10, wherein an N-terminal or a C-terminal amino acid tail on the class II E2 enzyme is replaced by the UBD.
 12. An E2 enzyme according to claim 10 or claim 11, wherein the Ubc domain is derived from UBE2S.
 13. An E2 enzyme according to claim 12, wherein the Ubc domain comprises residues 1 to 156 of UBE2S.
 14. A method for increasing the capacity of an E2 enzyme to produce free polyubiquitin chains in solution, comprising conjugating fusing the Ubc domain of said E2 enzyme to a UBD.
 15. A method according to claim 14, wherein the E2 enzyme is selected from the group consisting of UBE2A, UBE2B, UBE2C, UBE2D1, UBE2D2, UBE2D3, UBE2D4, UBE2E1, UBE2E2, UBE2E3, UBE2F, UBE2G1, UBE2G2, UBE2H, UBE21, UBE2J1, UBE2J2, UBE2K, UBE2L3, UBE2L6, UBE2M, UBE2N, UBE2NL, UBE20, UBE2Q1, UBE2Q2, UBE2R1, UBE2R2, UBE2S, UBE2T, UBE2U, UBE2V1, UBE2V2, UBE2V3, UBE2W, UBE2Z, AKTIP and BIRC6 and the UBD is a domain selected from the group consisting of UIM, IUIM (MIU), DUIM, UBM, UBA, GAT, CUE, VHS, UBZ, NZF, A20-like ZnF, ZnF UBP (PAZ), PRU, GLUE, UEV, UBC, SH3, PFU and Jab1/MNP domains.
 16. A method according to claim 15, wherein the E2 enzyme is UBE2S.
 17. A method according to claim 14, wherein the UBD is a ZnF UBP domain.
 18. A method for producing free polyubiquitin chains linked through a desired lysine residue, comprising the steps of: (a) selecting an E2 enzyme which possesses the desired lysine residue specificity; (b) fusing the Ubc catalytic domain of said E2 enzyme to a UBD ubiquitin binding domain; and incubating the resulting chimeric protein with an E1 ubiquitin activating enzyme and monomeric ubiquitin.
 19. A method according to claim 18, wherein the incidence of undesired lysine linkages is reduced by including a linkage-specific deubiquitinase in the incubation. 